Stimulation of glucose transport by thyroid hormone in 3T3-L1 adipocytes: increased abundance of GLUT1 and GLUT4 glucose transporter proteins

In 3T3-L1 adipocytes we have examined the effect of tri-iodothyronine (T 3 ) on glucose transport, total protein content and subcellular distribution of GLUT1 and GLUT4 glucose transporters. Cells incubated in T 3 -depleted serum were used as controls. Cells treated with T 3 (50 nM) for three days had a 3.6-fold increase in glucose uptake (P<0.05), and also presented a higher insulin sensitivity, without changes in insulin binding. The two glucose carriers, GLUT1 and GLUT4, increased by 87% (P<0.05) and 90% (P<0.05), respectively, in cells treated with T 3 . Under non-insulin-stimulated conditions, plasma membrane fractions obtained from cells exposed to T 3 were enriched with both GLUT1 (3.29 ± 0.69 vs 1.20 ± 0.29 arbitrary units (A.U.)/5 μg protein, P<0.05) and GLUT4 (3.50 ± 1.16 vs 0.82 ± 0.28 A.U./5 μg protein, P<0.03). The incubation of cells with insulin produced the translocation of both glucose transporters to plasma membranes, and again cells treated with T 3 presented a higher amount of GLUT1 and GLUT4 in the plasma membrane fractions (P<0.05 and P<0.03 respectively). These data indicate that T 3 has a direct stimulatory effect on glucose transport in 3T3-L 1 adipocytes due to an increase in GLUT1 and GLUT4, and by favouring their partitioning to plasma membranes, The effect of T 3 on glucose uptake induced by insulin can also be explained by the high expression of both glucose transporters.